Massive flood scarred the surface of Mars less than 500 million years ago

Water carved channels at least 70 meters deep and 40 kilometers wide.

Mars clearly had a watery history, with strong evidence of flowing streams and even some indications that an ocean was present in the distant past. The fate of Mars' water isn't understood, but there's evidence that some of it may have gone underground and is currently circulating in the bedrock of the red planet.

A study being released by Science finds further evidence that some of Mars' underground waters have burst to the surface violently. Using radar imaging, a team of scientists has tracked a series of channels buried under more recent features and has followed them back toward the source. The imaging showed that the main channel was about 40 kilometers wide and at least 70 meters deep. That's roughly the same size as the features carved by the largest well-characterized floods on Earth.

The work involved a radar instrument called SHARAD on the Mars Reconnaissance Orbiter. The radar can penetrate some surface features, revealing several layers beneath—depending on how they reflect the incoming radiation. This turned out to be extremely useful at the feature in question, Marte Vallis. It's one of a number of features on the Martian surface that suggests catastrophic flooding, but it's quite young at 500 million years old, long after water was thought to be common on the Martian surface. Unfortunately, studying it is complicated by the fact that it has been buried by an even younger feature: volcanic eruptions that make it difficult to even identify the source of the flood waters.

With the radar, however, a number of features that are invisible on the surface became apparent. Images revealed a number of smaller branching channels surrounding some small islands, all cut into the bedrock and since buried under the lava. These were roughly 40 meters deep, and the authors suggest they were carved during the early stage of the flooding.

That early stage later resolved into a single huge channel that was cut even deeper. This feature is about 40 kilometers wide (combined, the whole system is roughly 100km in width). Here the authors estimate the waters carved out the rock to a depth of at least 70m, and possibly over 115m—it's hard to tell, because it cut entirely through the bedrock and into a layer below that doesn't reflect radar well. At a minimum, that's twice the depth that had been previously estimated for Marte Vallis.

The authors note that the only confirmed flood feature of similar magnitude on Earth is also the largest we know of: the draining of Lake Missoula, which broke through a glacial dam and wiped out a lot of the soil of the eastern side of Washington during the last glacial period.

On its own, this was an impressive result, but the radar data also extended the known extent of the channels another 100km (up from 1,000km). That was enough to suggest the site that was the origin of the massive outburst of water, a feature called Cerberus Fossae. Whatever triggers these events, they tend to empty the source area of so much water that the surface terrain collapses, creating what's termed "chaos terrain." Apparently, Cerebrus Fossae fits the bill; it's a series of parallel faults that were thought to have been caused by volcanic activity.

Given the magnitude of the water involved, the obvious question is what's going on underneath Mars to collect it all in one place. Given that Cerebrus Fossae has been volcanically active, that's one reasonable guess as to what expelled the water.